This invention relates to modified phenol-formaldehyde resins, their preparation, and their use in adhesive compositions for bonding plywood. More particularly, this invention relates to resins prepared by the reaction of phenol with formaldehyde in the presence of (preferably) alkali treated bark or untreated bark and alkali.
Phenol-formaldehyde resins have long been used in the preparation of adhesives for use in the wood industry such as, for example, in the manufacture of plywood, particle-board and hardboard. As a means of improving the quality of adhesives, while reducing their cost in order to improve the economics of the production of plywood and the like, the use of extenders has been developed. These extenders have included lignosulfonates, and bark. In general, most of the modified phenol-formaldehyde resins containing extenders were prepared by a process in which the extenders were added after the initial reaction of the phenol and formaldehyde to form polymethylolphenols. These processes are essentially two step operations whereby (1) the polymethylolphenol was prepared by the reaction of phenol and formaldehyde under alkali conditions and (2) the resin was modified by the addition of the appropriate extender. Such a process is described, for example, in U.S. Pat. No. 3,213,045. The fact that this process involves two steps reduces somewhat the economic attractiveness of employing extenders.
U.S. Pat. No. 2,675,336 describes a process of preparing a modified phenol-formaldehyde resin in which a redwood bark extract powder is admixed with the phenol and formaldehyde reactants. The redwood bark product employed is an extract obtained by treatment of bark with a suitable alkaline product and subsequent separation from the other components of the bark. This is therefore also essentially a two step process, in which the first step is the treatment of the bark.
U.S. Pat. No. 3,371,054 discloses modified phenol-formaldehyde resins which employ a bark derived product as an extender. The bark derived product is obtained by treating the bark at very high temperature with a high percentage of alkali in an inert atmosphere, separating out the fines, and then acidifying the bark to produce an "acid bark". The acid bark is then rendered alkaline again and then reacted with formaldehyde to produce a formaldehyde-acid bark intermediate product. This intermediate product can then be further reacted with additional phenol to produce a novolak-type resin. Thus, the modified phenol-formaldehyde resins taught by this patent also involve the use of a multi-step process.
In U.S. Pat. No. 4,045,386, I have described a process for preparing resinous adhesives comprising alkali treated bark, tannin and aldehydes. The process described is an in situ production of adhesives involving the condensation of tannin and aldehydes (notably para-formaldehyde) in the presence of finely divided bark and other adhesive constituents. Because of the presence of the various other adhesive constituents designed for use with specified wood veneers, the product and process described in U.S. Pat. No. 4,045,386 is limited to use only with western mill veneers such as Douglas fir. It does not provide a resin which can be used to prepare adhesives which will successfully bond all veneers used in the manufacture of plywood.